This invention relates to the use of unique high solids maltodextrin syrups in adhesives, particularly remoistenable adhesives such as envelope adhesives and non-remoistenable adhesives such as bookbinding adhesives.
Starches and dextrins have long been used as an adhesive material in various applications such as the fabrication of corrugated board, paper bags, paper boxes, laminated paperboard, spiral-wound tubes, gummed labels, gummed tapes and other gumming applications. See the discussion in Starch Chemistry and Technology, 2nd Edition, by R. Whistler et al., 1984, pp. 593-610 and Chapter 22 "Starch and Its Modifications" by M. W. Rutenberg, pp. 22-63 and 22-64 in "Handbook of Water-Soluble Gums and Resins" edited by Robert L. Davidson and published by McGraw-Hill Book Co. (1980).
Typical adhesive categories include liquid adhesives, pastes, cold-water-soluble adhesives, water-resistant adhesives, and numerous other applications. In most applications, the starch is cooked and solubilized and used as either the only component in addition to the water in the adhesive or as an added component in more complex formulations to provide the required tack, overall adhesion, solution viscosity, stability, and/or desired rheological characteristics.
Adhesives for specific applications include corrugating adhesives, multiwall bag adhesives, laminating adhesives, tube-winding adhesives, labelling adhesives, side bag seam adhesives, tissue and towel adhesives, cigarette adhesives, wallpaper adhesives, adhesives for disposables, remoistenable adhesives, bookbinding adhesives, cup and plate adhesives, case and carton seal adhesives, carton forming adhesives, glued lap adhesives, and the like.
Remoistenable adhesives are commonly utilized in the production of envelopes, stamps, wallpaper, tapes, labels and similar products where it is desired to provide a dry, but remoistenable, adhesive film. In the manufacture of remoistenable adhesives, typically an aqueous solution or dispersion or an organic solvent solution of the dry adhesive material is applied as a wet film to a substrate. After the water or organic solvent is removed by drying and evaporation, the resulting dry adhesive film will, on being moistened, produce the desired tacky adhesive surface.
The ability to provide an adhesive which is remoistenable is not in itself difficult and, in fact, there are a number of known and commercially available products that are used in various applications requiring remoistenable adhesives. Conventional remoistenable adhesives have generally been prepared from either of two adhesive systems. The first class includes adhesives prepared by the addition of dextrin, plasticizer, and other additives to dextrin-emulsified vinyl acetate homopolymers. The second class includes polyvinyl acetate homopolymers and copolymers which are emulsified with polyvinyl alcohol or dextrin to which additional polyvinyl alcohol or dextrin and plasticizer may be post-added.
The usefulness of a particular composition as a remoistenable adhesive in a specific application will depend on its ability to meet the many physical and chemical property requirements and to best satisfy the processing conditions to which it is subjected. Accordingly, a remoistenable adhesive composition in addition to providing good adhesion and remoistenability, typically is expected to provide a lay flat or non-curl product, be processable on different manufacturing equipment, provide adequate drying speed, resist hygroscopic and thermal blocking, and satisfy various other requirements depending on the application involved.
Remoistenable adhesives based on dextrins and/or modified starches now require a tradeoff in desirable properties such as adequate adhesion and tack, high solids content (60-70%), and stable useable final viscosity (2500-25,000 cps). Typically, a high molecular weight starch or dextrin is required for adhesion but a lower molecular weight starch or dextrin is required for viscosity and stability.
An adhesive is stable if it remains at a near constant viscosity after manufacture. Most adhesives are utilized within a year of manufacture. If the adhesive increases or decreases in viscosity over time, the machining characteristics will be adversely affected as well as the storage handling and/or shelf life. If the viscosity increases over time, pasting or non-flow will occur, the adhesives will no longer function properly and poor coating will result. Flow problems can also occur when the adhesive is removed from the storage containers which can range in size from 5 to 7000 gallons or more.
One method of "stabilizing" a starch- or dextrin-based adhesive is by heating the cooked dextrin or starch in the presence of formaldehyde or glyoxal to effect a light crosslinking which minimizes retrogradation of the starch or dextrin (i.e., loose hydrogen bonding), which is commonly referred to as "pasting".
An inherent problem exists because higher molecular weight dextrins provide excellent adhesion, but the adhesives are extremely high in viscosity and have poor viscosity stability, whereas lower viscosity adhesives contain excessive water (added to achieve acceptable machine viscosities) but, as a result, the overall adhesive solids are lowered. The use of low solid adhesives also results in slow machine speeds since the coated adhesive must be dried before packaging. In contrast, low molecular weight dextrins provide low viscosity, high solids adhesives which give poor adhesion and poor tack on remoistening.
Humidity resistance is very important when formulating a remoistenable adhesive. The adhesive should not be prematurely reactive when exposed to normal atmospheric conditions.
"Blocking" is the undesirable adhesion that can occur between touching layers of a material under certain conditions. Hygroscopic blocking resistance is the ability of the adhesive to resist reactivating due to humidity. In a situation where substrates, such as envelopes or sheets of stamps, are coated with an adhesive which is non-resistant to hygroscopic blocking and are stored and exposed to varying degrees of humidity, there is a tendency for adjacent surfaces of the stacked substrates to adhere and stick to one another. Thermoplastic blocking is the adhesion that can occur when adhesive coated substrates are exposed to varying temperatures and pressures during processing, storage and other handling operations. Pressure conditions causing blocking can exist, for example, during storage or stacking of products while processing, during the printing of products such as the laser jet printing of papers and envelopes, and during the perforating of sheets of stamps and other products. Depending on the particular conditions used, the amount of pressure that is created can cause blocking in non-resistant adhesive formulations.
Hygroscopic blocking has been more particularly defined by a standard test method, ASTM D 114653. Hygroscopic blocking is measured on a test specimen at 50 percent relative humidity at 38.degree. C. for 24 hours. If there is no blocking (free), it is then measured at increments of successively higher humidities until blocking occurs (critical humidity) or until a suitably high humidity is reached. Thermoplastic blocking is measured under the same. ASTM D procedure at 38.degree. C. for 24 hours. If there is no blocking (free), it is measured at successively higher temperatures (increments of 5.degree. C.) until blocking occurs (critical temperature) or until a suitably high temperature is reached. Especially suitable high humidity and high temperature conditions for determining hygroscopic and thermoplastic blocking are 95 percent relative humidity and 90.degree. C.
Since remoistenable adhesives are mainly used on paper substrates, the use of excessive water can also result in poor lay-flat or "curling" of the coated surface. The paper absorbs water and swells, distorting the original configuration of the paper fibers. As the adhesive dries, wrinkling or "curling" occurs due to the unequal rates of expansion and contraction of the paper backing and adhesive film.
Excessive curl can cause equipment jam-ups or other handling difficulties. While some adhesive materials have good lay flat or non-curl properties, others require various techniques such as the addition of humectants to overcome or minimize this curling tendency. While these techniques often alleviate the curling problem, other properties such as hygroscopic blocking resistance can be unfavorably affected.
To solve this problem, one can reduce the overall amount of water in the final adhesive formulation or add humectants which act as diluents and do not distort the paper fibers. The use of humectants, however, increases the time required to dry the adhesive. The use of humectants also reduces the adhesive's humidity resistance since the humectants are hygroscopic.
Pyrodextrins are used in the preparation of many liquid and dry adhesives including remoistenable adhesives. There are four major steps in the manufacture of pyrodextrins: acidification, predrying, dextrinization, and cooling. In the early stages, hydrolysis is the major reaction and the viscosity of the starch is substantially reduced to near the level of the finished dextrin. Repolymerization becomes a major factor as the temperature rises. As the reaction progresses, an equilibrium viscosity is approached, and at increasing temperatures, a transglucosidation reaction predominates. There are two major characteristic changes--one is the molecular size of the dextrin molecule, the other is a change in the degree of linearity. Each has a specific effect on the physical and chemical characteristics of the dextrin. Variation in average molecular weight influences the dextrin's viscosity, while changes in linearity greatly influence the solution stability.
The use of pyrodextrins in remoistenable adhesives requires compromises in adhesive formulating between the desired solids, viscosity, tack/strength and stability. In addition, the adhesives suffer from color problems.
Bookbinding adhesives require superior adhesion and various classes of adhesives are utilized in the bookbinding industry. For example, synthetic emulsion adhesives, such as polyvinyl alcohol or ethylene vinyl acetate emulsion polymers, are used on endpapers, for tipping, for gluing-off operations, for lining, casing-in, spine gluing and side gluing, and on special stencil applicator case makers. Hot melt adhesives, which consist of polymers, tackifier resins and waxes, are used mainly as one-shot adhesives for pocket-books and magazines, in multi-shot applications in the production of catalogues, and in burst binding. Starches derived from maize, potato, and tapioca are used for producing pastes, generally for the hand-binding section of the trade and sometimes for operations such as endpapering and tipping or for hand case making in leather.
Case making is one of the more demanding bookbinding techniques. Because of the adhesive properties required, animal glues are mainly used for case making and lining and only occasionally in unsewn binding lines. Animal glues consist of one or more kinds of gelatine in a solution of water. They give aggressive tack with a medium-set speed. Since they are derived from natural materials, they are very susceptible to seasonal factors and tend to vary with their source of supply. Operators using these glues have to be skilled in achieving the correct degree of dilution and tack. Most animal glues age and become brittle under dry, warm conditions as evidenced by the condition of the spine of a book kept for some time in a centrally heated home.
Most cigarette making adhesives are based on synthetic polymer systems. There is, however, a growing trend to the use of natural products in this area. Modified starches, such as chemically modified starches and dextrins, are being used for such constructions. Adhesives containing these starches and dextrins have a Brookfield viscosity of approximately 500-5000 cps., and they are used in all applications including side seams and tipping. A side seam is the bond produced to facilitate the formation of the tobacco filled cigarette rod. Tipping is the process by which the separate filter section is combined with the tobacco filled section by means of an overlapping bond.
Corrugating adhesives are prepared using starch, water, alkali, and other optional ingredients, e.g., a waterproofing agent. Starches used as the carrier portion may contain portions or mixtures of high amylose starch.
There is a need for improved maltodextrin-based adhesives having a high adhesive solids content, which maintain a useable viscosity during storage, and which provide good adhesion and which also provide good lay flat properties, block resistance, and tack when the adhesive is a remoistenable adhesive and resistance to humidity changes when the adhesive is a bookbinding adhesive.